Predistortion circuit, low-distortion power amplifier, and control methods therefor

ABSTRACT

An amplitude frequency characteristic adjustment circuit  106  is provided downstream of and connected to a distortion generation circuit  105 . An amplitude difference between low-frequency-side and high-frequency-side distortion voltages is adjusted by the amplitude frequency characteristic adjustment circuit  106 , and then their amplitudes and phases are adjusted by a vector adjustment circuit  107 . This configuration makes it possible to suppress simultaneously both of low-frequency-side and high-frequency-side distortion voltages of a distortion generated by a wide-band class-AB power amplifier even if they are different in amplitude and phase.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of Ser. No. 10/946,301, filed Sep. 22,2004, which is a division of Ser. No. 10/750,910, filed Jan. 5, 2004,which is a division of Ser. No. 10/395,092, filed Mar. 25, 2003, whichis a division of Ser. No. 09/865,692, filed May 29, 2001 (now U.S. Pat.No. 6,590,449), all of which are being incorporated in their entiretyherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to linearization of a power amplifier usedin a cellular phone and base station, for example, as well as to alow-distortion power amplifier and the like.

2. Description of the Related Art

FIG. 17 shows a conventional predistortion circuit. As shown in FIG. 17,a signal that is input to an input terminal 1701 is divided to tworoutes by a divider circuit 1703. In the first route, the signal isinput to a combiner 1707 via a delay circuit 1704. On the other hand, inthe second route, a distortion is generated by a distortion generationcircuit 1705 and the signal is input to the combiner 1707 via a vectoradjustment circuit 1706. The two signals are combined with each other bythe combiner 1707 and a resulting signal is output from an outputterminal 1702 and supplied to a power amplifier (not shown). Thepredistortion circuit generates a signal for suppressing a distortion ofthe downstream power amplifier by varying the amplitude and phase of adistortion with the vector adjustment circuit 1706, whereby a distortionat the output of the power amplifier is suppressed. The delay time ofthe delay circuit 1704 is so set as to equalize the delay times of thefirst and second routes.

However, in particular, where a transmission signal has a wide bandwidthand the power amplifier performs a class-AB operation, the amplitude andphase characteristics of a intermodulation distortion generated by theamplifier lose balance and hence the effect of linearization (forexample, suppressing distortion) degrades.

A more detailed description will be made below. It is assumed that thepower amplifier is matched in a wide band and that the gain and the passphase have no deviations in a transmission band. When two signals havingdifferent frequencies and the same amplitude are input, the outputvoltage V_(O) is given byV _(O) =A _(O)(cosω₁ t+cosω₂ t)+B _(OL) cos[(2ω₁−ω₂)t+φ_(3L) ]+B _(OU)cos[(2ω₂−ω₁)t+φ _(3U)]  [Equation 1]where ω₁ and ω₂ are the angular frequencies of the input signals, A_(O)is the amplitude component of voltages having angular frequencies ω₁ andω₂ (among the output voltages), B_(OL) and B_(OU) are the amplitudecomponents of third-order intermodulation distortion voltages occurringon the low-frequency side and the high-frequency side, and φ_(3L) andφ_(3U) are the phase components of the third-order intermodulationdistortion voltages occurring on the low-frequency side and thehigh-frequency side.

In this case, both intermodulation distortions occurring on thelow-frequency side and the high-frequency side can be suppressed for bygenerating a voltage VI given by the following equation with thelonearizer (for example, predistortion circuit) and inputting it to thepower amplifier:V _(I)=A_(I)(cosω₁ t+cosω₂ t)−B_(IL) cos[(2ω₁−ω₂)t+φ _(3L)]−B_(IU)cos[(2ω₂−ω₁)t+φ_(3U)]  [Equation 2]where A_(I) is the amplitude component of voltages having angularfrequencies ω₁ and ω₂ and B_(IL) and B_(IU) are the amplitude componentsof third-order intermodulation distortion voltages occurring on thelow-frequency side and the high-frequency side in the predistortioncircuit. There are relationships A_(O)=A_(I)·G, B_(OL)=B_(IL)·G, andB_(OU)=B_(IU)·G, where G is the voltage gain of the power amplifier.

According to the conventional technique, the amplitudes and the phasesof third-order intermodulation distortion occurring in the predistortioncircuit cannot be controlled independently on the low-frequency side andthe high-frequency side. That is, B_(IL), B_(IU), φ_(3L), and φ_(3U)cannot be controlled independently.

No particular problems occur with the above conventional linealizer ifthe amplitude components of third-order intermodulation distortionvoltages on the low-frequency side and the high-frequency side are thesame and their phase components are also the same.

However, if the amplitude components and/or the phase components ofthird-order intermodulation distortion voltages on the low-frequencyside and the high-frequency side are different from each other as in thecase that a transmission signal has a wide bandwidth and the poweramplifier performs a class-AB operation, a problem arises that asufficient linearization (for example, sufficient suppressingdistortion) cannot be obtained.

BRIEF SUMMARY OF THE INVENTION

Object of the Invention

The present invention has been made in view of the above problems in theart, and an object of the invention is therefore to provide apredistortion circuit, a low-distortion power amplifier, control methodstherefor, etc. in which at least one of the amplitudes and the phases ofa distortion on the high-frequency side and the low-frequency side arecontrolled independently and that are therefore effectively used for apower amplifier having such an unbalanced distortion characteristic.

The 1st invention of the present invention is a predistortion circuitcomprising:

-   -   a divider for branching an input signal into plural branched        signals;    -   a delay circuit for delaying one of the branched signals by a        predetermined delay time;    -   a distortion generating circuit for receiving the other branched        signal and for generating a distortion signal;    -   a vector adjustment circuit for varying an amplitude and a phase        of the distortion signal; and    -   a combining circuit for combining an output signal of the delay        circuit and an output signal of the vector adjustment circuit        and for outputting a combined signal to circuit means as a        subject of linearization to be provided downstream of and        connected directly or indirectly to the combining circuit,    -   wherein the delay time of the delay circuit is set based on a        phase difference of a distortion that would be generated by the        circuit means if the vector adjustment circuit did not produce        the output signal.

The 2nd invention of the present invention is a predistortion circuitcomprising:

-   -   a divider for branching an input signal into plural branched        signals;    -   a delay circuit for delaying one of the branched signals by a        predetermined delay time;    -   a distortion generating circuit for receiving the other branched        signal and for generating a distortion signal;    -   an amplitude frequency characteristic adjustment circuit for        varying an amplitude frequency characteristic of the distortion        signal;    -   a vector adjustment circuit for varying an amplitude and a phase        of the distortion signal that is output from the amplitude        frequency characteristic adjustment circuit; and    -   a combining circuit for combining an output signal of the delay        circuit and an output signal of the vector adjustment circuit.

The 3rd invention of the present invention is the predistortion circuitaccording to 2nd invention, wherein the delay time of the delay circuitis set based on a phase difference of a distortion that would begenerated by circuit means as a subject of linearization to receive anoutput signal of the combining circuit if the vector adjustment circuitdid not produce the output signal.

The 4th invention of the present invention is the predistortion circuitaccording to 1st or 3rd inventions, wherein the delay time of the delaycircuit set based on the phase difference in such a manner that a firstdelay time is so set that a difference between the first delay time anda second delay time substantially equivalent to or corresponding to thephase difference, where the first delay time is the delay time itselfand the second delay time is a delay time of a signal that is input tothe combining circuit via the distortion generation circuit and thevector adjustment circuit.

The 5th invention of the present invention is the predistortion circuitaccording to 1st or 3rd inventions, wherein the delay time of the delaycircuit is variable.

The 6th invention of the present invention is the predistortion circuitaccording to 1st or 3rd inventions, wherein the delay time of the delaycircuit is fixed at a predetermined value.

The 7th invention of the present invention is a predistortion circuitcomprising:

-   -   a divider for branching an input signal into plural branched        signals;    -   a delay circuit for receiving one of the branched signals;    -   a distortion generating circuit for receiving the other branched        signal and for generating a distortion signal;    -   at least two filter circuits for separating the distortion        signal into distortion signals having different frequencies;    -   at least two vector adjustment circuits connected to outputs of        the respective filter circuits directly or indirectly, for        varying amplitudes and phases of the distortion signals that are        output from the respective filter circuits; and    -   a combining circuit for combining an output signal of the delay        circuit and combined output signals of the respective vector        adjustment circuits.

The 8th invention of the present invention is the predistortion circuitaccording to 7th invention, further comprising at least two amplitudefrequency characteristic adjustment circuits for varying amplitudefrequency characteristics of distortion signals that are output from therespective filter circuits, wherein the vector adjustment circuits areconnected to outputs of the respective amplitude frequencycharacteristic adjustment circuits.

The 9th invention of the present invention is the predistortion circuitaccording to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, whereinthe distortion generation circuit comprises a limiter amplifier.

The 10th invention of the present invention is the predistortion circuitaccording to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, whereinthe distortion generation circuit comprises a diode.

The 11th invention of the present invention is the predistortion circuitaccording to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, whereinthe distortion generation circuit comprises a zero-bias diode.

The 12th invention of the present invention is the predistortion circuitaccording to any one of 1st, 2nd, 3rd, 7th, and 8th inventions, whereinthe distortion generation circuit comprises:

-   -   a divider for branching an input signal into plural signals;    -   a delay circuit connected to one output side of the divider;    -   a circuit connected to the other output side of the divider and        comprising a nonlinear device;    -   a vector adjustment circuit connected to an output side of the        circuit comprising the nonlinear device; and    -   a combining circuit for combining an output signal of the delay        circuit and an output signal of the vector adjustment circuit.

The 13th invention of the present invention is a low-distortion poweramplifier comprising:

-   -   a combining circuit for combining an input signal with another        signal;    -   a power amplifier for receiving an output signal of the        combining circuit;    -   a divider for branching an output signal of the power amplifier        into plural branched signals;    -   a distortion extraction circuit for extracting a distortion        signal from one of the branched signals;    -   an amplitude frequency characteristic adjustment circuit for        varying an amplitude frequency characteristic of the distortion        signal; and    -   a vector adjustment circuit for varying an amplitude and a phase        of the distortion signal that is output from the amplitude        frequency characteristic adjustment circuit,    -   wherein an output signal of the vector adjustment circuit is        input to the combining circuit as said another signal and the        other branched signal is output from the low-distortion power        amplifier.

The 14th invention of the present invention is a low-distortion poweramplifier comprising:

-   -   a combining circuit for combining an input signal with another        signal;    -   a power amplifier for receiving an output signal of the        combining circuit;    -   a divider for branching an output signal of the power amplifier        into plural branched signals;    -   a distortion extraction circuit for extracting a distortion        signal from one of the branched signals;    -   at least two filter circuits for separating the distortion        signal into distortion signals having different frequencies; and    -   at least two vector adjustment circuits for varying amplitudes        and phases of the distortion signals that are output from the        respective filter circuits,    -   wherein a signal obtained by combining together output signals        of the vector adjustment circuits is input to the combining        circuit as said another signal and the other branched signal is        output from the low-distortion power amplifier.

The 15th invention of the present invention is a low-distortion poweramplifier comprising:

-   -   a combining circuit for combining an input signal with another        signal;    -   a power amplifier for receiving an output signal of the        combining circuit;    -   a divider for branching an output signal of the power amplifier        into plural branched signals;    -   a distortion extraction circuit for extracting a distortion        signal from one of the branched signals;    -   at least two filter circuits for separating the distortion        signal into distortion signals having different frequencies;    -   at least two amplitude frequency characteristic adjustment        circuits for adjusting amplitude frequency characteristics of        the distortion signals that are output from the respective        filter circuits; and    -   at least two vector adjustment circuits for varying amplitudes        and phases of distortion signals that are output from the        respective filter circuits,    -   wherein a signal obtained by combining together output signals        of the vector adjustment circuits is input to the combining        circuit as said another signal and the other branched signal is        output from the low-distortion power amplifier.

The 16th invention of the present invention is a control method for thepredistortion circuit according to any one of 1st, 2nd, 3rd, 7th, and8th inventions, comprising the steps of:

-   -   connecting a power amplifier to an output side of the        predistortion circuit;    -   detecting a magnitude of a distortion signal generated by the        power amplifier; and    -   controlling at least one of the amplitude frequency        characteristic adjustment circuit or circuits, the vector        adjustment circuit or circuits, and the delay time of the delay        circuit so as to minimize the detected magnitude of the        distortion signal.

The 17th invention of the present invention is the control method forthe low-distortion power amplifier according to any one of 13th to 15thinventions, comprising the steps of:

-   -   detecting a magnitude of the distortion signal that is output        from the distortion extraction circuit; and    -   controlling at least one of the amplitude frequency        characteristic adjustment circuit or circuits and the vector        adjustment circuit or circuits so as to minimize the detected        magnitude of the distortion signal.    -   The 18th invention of the present invention is a linearized        power amplifier comprising:    -   the predistortion circuit according to any one of 1st, 2nd, 3rd,        7th, and 8th inventions;    -   a power amplifier for receiving an output signal of the        predistortion circuit;    -   a divider for branching an output signal of the power amplifier        into plural branched signal;    -   detecting means of receiving one of the branched signals and        detecting an amplitude and a phase of a distortion signal that        is output from the power amplifier; and    -   control means of controlling at least one of the amplitude        frequency characteristic adjustment circuit or circuits, the        vector adjustment circuit or circuits, and the delay circuit of        the predistortion circuit so as to minimize a distortion        generated by the power amplifier based on an output signal of        the distortion amplitude and phase detecting means,    -   wherein the other branched signal becomes at least one output        signal of the linearized power amplifier.

The 19th invention of the present invention is a feedforward amplifierhaving a predistortion circuit, comprising:

-   -   a divider for branching an input signal into plural branched        signals;    -   a first vector adjustment circuit for varying an amplitude and a        phase of one of the branched signals;    -   the predistortion circuit according to any of the 1st, 2nd, 3rd,        7th, and 8th inventions for receiving an output signal of the        first vector adjustment circuit;    -   a first power amplifier for receiving an output signal of the        predistortion circuit;    -   first distortion level detecting means of detecting a magnitude        of a distortion component included in an output signal of the        first power amplifier;    -   a first delay circuit for receiving the other branched signal;    -   a first combining circuit for combining an output signal of the        first delay circuit and the output signal of the first power        amplifier;    -   a second delay circuit for delaying the output signal of the        first power amplifier;    -   signal level detecting means of detecting a magnitude of an        output signal of the first combining circuit;    -   a second vector adjustment circuit for varying an amplitude and        a phase of the output signal of the first combining circuit;    -   a second power amplifier for receiving an output signal of the        second vector adjustment circuit;    -   a second combining circuit for combining an output signal of the        second power amplifier and an output signal of the second delay        circuit;    -   second distortion level detecting means of detecting a magnitude        of a distortion component included in an output signal of the        second combining circuit; and    -   control means of controlling the predistortion circuit, the        first vector adjustment circuit, and the second vector        adjustment circuit based on output signals of the first        distortion level detecting means, the signal level detecting        means, and the second distortion level detecting means,        respectively,    -   wherein the control means repeatedly performs, in arbitrary        order, a first control of controlling at least the predistortion        circuit so as to minimize a distortion level detected by the        first distortion level detecting means, a second control of        controlling at least the first vector adjustment circuit so as        to minimize a signal level detected by the signal level        detecting means, and a third control of controlling at least the        second vector adjustment circuit so as to minimize a distortion        level detected by the second distortion level detecting means.

The 20th invention of the present invention is the feedforward amplifierhaving a predistortion circuit according to 19th invention, wherein:

-   -   the first delay circuit is a variable delay circuit whose delay        time is variable;    -   a variation amount of a delay time when each of the first vector        adjustment circuit and the predistortion circuit was controlled        is stored in the control means; and    -   the control means controls the predistortion circuit and the        variable delay circuit as the first control, controls the first        vector adjustment circuit and the variable delay circuit as the        second control, and controls only the second vector adjustment        circuit as the third control.

The 21st invention of the present invention is a predistortion circuitcomprising:

-   -   a divider for branching an input signal into plural branched        signals;    -   a delay circuit for delaying one of the branched signals by a        predetermined delay time;    -   a distortion generating circuit for receiving the other branched        signal and for generating a distortion signal;    -   a vector adjustment circuit for varying an amplitude and a phase        of the distortion signal; and    -   an amplitude frequency characteristic adjustment circuit for        varying an amplitude frequency characteristic of the distortion        signal that is output from the vector adjustment circuit;    -   a combining circuit for combining an output signal of the delay        circuit and an output signal of the vector adjustment circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a predistortioncircuit according to a first embodiment of the present invention;

FIG. 2A shows how the predistortion circuit and a power amplifier areconnected to each other;

FIG. 2B illustrates a characteristic that the predistortion circuit isrequired to have;

FIGS. 3A–3D illustrate the principle of the predistortion circuitaccording to the invention;

FIG. 4A shows an example of a distortion generation circuit using adiode in the first embodiment of the invention;

FIG. 4B shows another example of the distortion generation circuit inthe first embodiment of the invention;

FIG. 5 shows an example of an amplitude frequency characteristicadjustment circuit in the first embodiment of the invention;

FIG. 6 shows an example of a vector adjustment circuit in the firstembodiment of the invention;

FIG. 7 is a block diagram of a predistortion circuit according to thefirst embodiment of the invention using a variable delay circuit;

FIG. 8 shows an example of a variable delay circuit in the firstembodiment of the invention;

FIG. 9 is a block diagram showing the configuration of a predistortioncircuit according to a second embodiment of the invention;

FIG. 10 is a block diagram showing the configuration of anotherpredistortion circuit according to the second embodiment of theinvention;

FIG. 11 is a block diagram showing the configuration of a furtherpredistortion circuit according to the second embodiment of theinvention;

FIG. 12 is a block diagram showing the configuration of a low-distortionpower amplifier according to a third embodiment of the invention;

FIG. 13 is a block diagram showing the configuration of herlow-distortion power amplifier according to a third embodiment of theinvention;

FIG. 14 is a block diagram showing the configuration of a furtherlow-distortion power amplifier according to a third embodiment of theinvention;

FIG. 15 is a block diagram showing the configuration of a linearizedpower amplifier according to a fourth embodiment of the invention;

FIG. 16 is a block diagram showing the configuration of a feedforwardamplifier according to a fifth embodiment of the invention; and

FIG. 17 is a block diagram showing the configuration of a conventionalpredistortion circuit.

DESCRIPTION OF SYMBOLS

-   101 Input terminal-   102 Output terminal-   103 Divider-   104 Delay circuit-   105 Distortion generation circuit-   106 Amplitude frequency characteristic adjustment circuit-   107 Vector adjustment circuit-   108 Combiner-   203 Predistortion circuit-   204 Power amplifier-   403, 408 Capacitor-   404 Power supply terminal-   405, 407 Inductor-   406 Diode-   413 Circuit including a nonlinear device-   503 Transmission line-   504 Variable capacitor-   603, 604 Directional coupler-   605, 606 Resistance with variable values-   704 Variable delay circuit-   1306 Distortion extraction circuit-   1506 Distortion amplitude/phase detecting means-   1507 Control means-   1508 Control terminal-   1608, 1619 Distortion level detecting means-   1614 Signal level detecting means

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

A first embodiment of the present invention will be hereinafterdescribed with reference to FIGS. 1–8.

Referring to FIG. 1, a signal that is input to an input terminal 101 isbranched into two signals by a divider 103. One branched signal is inputto a combiner 108 via a delay circuit 104. The other output signal ofthe divider 103 is input to a distortion generation circuit 105, where adistortion signal is generated. After the frequency characteristic ofthe amplitude of the distortion signal is varied by an amplitudefrequency characteristic adjustment circuit 106, the amplitude and thephase of the distortion signal are varied by a vector adjustment circuit107. A resulting signal is input to a combiner 108. A distortion of adownstream power amplifier can be suppressed for by using an output atan output terminal 102 of the combiner 108.

A specific example in which an input signal is two continuous waveshaving the same amplitude and different frequencies will be describedwith reference to FIGS. 1–3.

Referring to FIGS. 2A and 2B, a distortion generated by a poweramplifier 204 can be suppressed for by generating, with a predistortioncircuit 203, a distortion having the same amplitude (that is normalizedby the amplitude of a carrier) as the distortion generated by the poweramplifier 204 and the opposite phase (that is, defference between phasesof intermodulation distortion signals is 180 degrees) to the phase ofthe distortion generated by the power amplifier 204 and inputting it tothe power amplifier 204. The phase of distortion signal is defined asthe difference between θ₂ and θ₁ (that is, θ₂−θ₁), wherein the phasewhen the phases of two carrier signals become the same is defined as θ₁and the momentary phase of the distortion signal at that time is definedas θ₂. However, it is difficult to obtain desired amplitude and phase ofthe intermodualtion distortion signals directly. In view of this, thepredistortion circuit 203 is given the configuration of FIG. 1.

That is, an input terminal 201 corresponds to the input terminal 101 inFIG. 1 and the power amplifier 204 is connected to the output terminal102 in FIG. 1.

This embodiment will be described for a case where the amplitudecomponents and phase components of third-order intermodulationdistortion voltages of the power amplifier 204 on the low-frequency sideand the high-frequency side are different from each other and theirphase components are also different from each other, that is, the poweramplifier as a subject of linearization (for example, suppressingdistortion) and the predistortion circuit are different in both of theamplitude characteristic and the phase characteristic of the distortioncharacteristic.

As shown in FIG. 3A, it is assumed that third-order intermodulationdistortion voltages IM3 on the low-frequency side and the high-frequencyside generated by the distortion generation circuit 105 in FIG. 1 havethe same amplitude and phase. As shown in FIG. 3B, the amplitudefrequency characteristic adjustment circuit 106 adjusts the distortionvoltages on the low-frequency side and the high-frequency side so thattheir amplitude ratio becomes equal to a prescribed value. Then, asshown in FIG. 3C, the vector adjustment circuit 107 adjusts theamplitudes and the phases of the respective distortion voltages toprescribed values. Finally, as shown in FIG. 3D, the voltages ofintermodulation distortion signals are given a prescribed phasedifference θ by using a delay time that is set in advance by the delaycircuit 104.

A description will be made of the phase difference θ between distortionvoltages (see FIG. 3D) and the delay time of the delay circuit 104.

Referring to FIG. 1, the delay time of a signal that is input to thecombiner 108 via the delay circuit 104 is now called a first delay timeand the delay time of a signal that is input to the combiner 108 via thedistortion generation circuit 105, the amplitude frequencycharacteristic adjustment circuit 106, and the vector adjustment circuit107 is now called a second delay time. The first delay time is set inadvance in the designing stage of the predistortion circuit so that thedifference between the first and second delay times becomessubstantially equal to the above-mentioned phase difference θ. Theprescribed value θ is equal to the difference between the phasecomponents of third-order intermodulation distortion voltage on thelow-frequency side and the high-frequency side generated by the poweramplifier 204 (the difference between the phase components is defined asthe difference between the phases of a low-frequency-side distortionvoltage and a high-frequency-side distortion voltage with respect to thesame phase of two carrier signals).

The predistortion circuit 203 is given the desired characteristic inthis manner and the distortion of the power amplifier 204 can thereby besuppressed for.

Although the distortion generation circuit 105 may be formed by using alimiter amplifier, a configuration shown in FIG. 4A is also possiblethat uses a diode. In the configuration of FIG. 4A, a signal is input toan input terminal 401 and output from an output terminal 402. A diode406 is biased being supplied with a voltage from a power supply terminal404. A configuration that does not require supply of power is possibleby making the diode 406 a zero-bias diode and grounding the power supplyterminal 404.

FIG. 4B shows another configuration of the distortion generation circuit105. As shown in FIG. 4B, a signal that is input to an input terminal409 is branched (devided) into two signals by a divider 411. Onebranched (devided) signal is input to a combiner 415 via a delay circuit412. The other branched (devided) signal is input to a circuit 413including a nonlinear device and a signal including a distortioncomponent is output from the circuit 413. The amplitude and the phase ofthe output signal of the circuit 413 are adjusted by a vector adjustmentcircuit 414 and a resulting signal is input to the combiner 415. Bycausing the two carrier signals that are input to the combiner 415 tohave the same amplitude and a difference of 180° between phases of thesesignals, carrier components are suppressed and only a distortioncomponent is obtained at the output of the combiner 415.

For example the amplitude frequency characteristic adjustment circuit106 may be a circuit shown in FIG. 5. Referring to FIG. 5, the amplitudefrequency characteristic can be varied by varying the capacitance valuesof a capacitor 504.

For example, the vector adjustment circuit 107 may be a circuit shown inFIG. 6. Referring to FIG. 6, the amplitude attenuation amount can bevaried by changing the resistance values of resistors 605 and 606 andthe pass phase can be varied by changing the capacitance values ofcapacitors 607 and 608.

By employing, as the delay circuit 104 (see FIG. 1), a variable delaycircuit 704 (see FIG. 7) whose delay time is variable, the phasedifference between distortion voltages on the low-frequency side and thehigh-frequency side can be adjusted.

FIG. 7 shows a predistortion circuit of this case. FIG. 8 shows anexample of the variable delay circuit 704.

By switching among transmission lines 805–807 using switches 803 and804, the delay time can be varied.

For example, even if the characteristic of a downstream amplifier isdetermined, preparing a plurality of delay times in this manner enablesfine adjustment of the delay time, which in turn makes it possible toabsorb a deviation in adjustment of the delay time due to variations inthe characteristic of the amplifier and other circuits. Even if thereare a plural kinds of characteristics of a downstream amplifier andhence the characteristic cannot be determined in advance, preparing aplurality of delay times in advance enables selection of a delay timesuitable for the characteristic of an amplifier that is actuallyconnected to the predistortion circuit.

In this case, the amplitude frequency characteristic adjustment circuit106 or 706 can be omitted if the ratio of the magnitude of twodistortion signals generated by the distortion generating circuit andthe ratio of the magnitude of two distortion signals the power amplifierare equal.

Highly accurate suppressing distortion can be attained, in a stablemanner by extracting all or a part of distortion component of a poweramplifier connected to the output of this predistortion circuit andcontrolling at least one of the delay circuit 104 (delay time), theamplitude frequency characteristic adjustment circuit 106, and thevector adjustment circuit 107 so as to minimize the magnitude of theextracted distortion component.

Although the above description is directed to the adjustment of theamplitudes and the phases of third-order intermodulation distortionvoltages on the low-frequency side and the high-frequency side, similareffects can also be obtained for the adjustment of the amplitudes andthe phases of a third-order intermodulation distortion voltage and afifth-order intermodulation distortion voltage.

This embodiment also provides an effect of suppressing a distortion thatoccurs when a modulated wave is input.

Embodiment 2

A second embodiment of the invention will be hereinafter described withreference to FIGS. 9 and 10.

As shown in FIG. 9, a signal that is input to an input terminal 901 isbranched into two signals by a divider 903. One branched signal is inputto a combiner 910 via a delay circuit 904. The other output of thedivider 903 is input to a distortion generation circuit 905, where adistortion signal is generated.

Also in this embodiment, a specific example will be described in whichan input signal is two continuous waves having the same amplitude anddifferent frequencies.

A filter circuit 906 selectively passes only a third-orderintermodulation distortion voltage on the low-frequency side of athird-order intermodulation distortion generated by the distortiongeneration circuit 905 and attenuates the other frequency component. Afilter circuit 907 selectively passes only a third-order intermodulationdistortion voltage on the high-frequency side and attenuates the otherfrequency components.

Of the third-order intermodulation distortion generated by thedistortion generation circuit 905, the third-order intermodulationdistortion voltage on the low-frequency side passes through the filtercircuit 906 and its amplitude and phase are varied by a vectoradjustment circuit 908. The third-order intermodulation distortionvoltage on the high-frequency side passes through the filter circuit 907and its amplitude and phase are varied by a vector adjustment circuit909. Outputs of the vector adjustment circuits 908 and 909 are combinedtogether and then input to a combiner 910. Since in this mannerthird-order intermodulation distortion voltages on the low-frequencyside and the high-frequency side can be controlled independently, adistortion of a power amplifier having an unbalanced distortioncharacteristic can be suppressed for. This embodiment also provides aneffect of suppressing a distortion that occurs when a modulated wave isinput.

This embodiment is different from the first embodiment in that thevector adjustment circuits 908 and 909 are adjusted so that thedifference between the phase of a distortion that is output from thevector adjustment circuit 908 with respect to the phase of two carriersignals having the same phase and the phase of a distortion that isoutput from the vector adjustment circuit 909 with respect to phase ofthose carrier signals becomes equal to the above-mentioned prescribedvalue θ.

Highly accurate suppressing distortion can be attained in a stablemanner by extracting all or a part of a distortion component of a poweramplifier connected to the output of this predistortion circuit andcontrolling at least one of the vector adjustment circuits 908 and 909so as to minimize the magnitude of the extracted distortion component.

Although the configuration of FIG. 9 is directed to the adjustment ofthe amplitudes and the phases of third-order intermodulation distortionvoltages on the low-frequency side and the high-frequency side, theinvention is not limited to such a case. For example, a configuration ofFIG. 10 is also possible.

Referring to FIG. 10, a filter circuit 1006 selectively passes athird-order intermodulation distortion voltage on the low-frequency sideand attenuates the other frequency components. A filter circuit 1007selectively passes a third-order intermodulation distortion voltage onthe high-frequency side and attenuates the other frequency components. Afilter circuit 1008 selectively passes a fifth-order intermodulationdistortion voltage on the low-frequency side and attenuates the otherfrequency components. A filter circuit 1009 selectively passes afifth-order intermodulation distortion voltage on the high-frequencyside and attenuates the other frequency components.

This configuration makes it possible to independently control theamplitudes and the phases of third-order intermodulation distortionvoltages and the amplitudes and the phases of fifth-orderintermodulation distortion voltages, and thereby provides the sameeffect of suppressing distortion as in the above example.

A configuration shown in FIG. 11 is also possible.

As shown in FIG. 11, a distortion signal generated by a distortiongeneration circuit 1105 is frequency-separated by filter circuits 1106and 1107.

For example, where a plurality of distortion signals are included inoutputs of the filter circuits 1106 and 1107, those distortion signalsare subjected to adjustment of the amplitude frequency characteristic inamplitude frequency characteristic adjustment circuits 1108 and 1109 andamplitude and phase adjustment in vector adjustment circuits 1110 and1111. Resulting signals are input to a combiner 1112, where it iscombined with a signal that went through a delay circuit 1104.

This configuration enables suppressing distortion even on distortioncomponents having a plurality of frequencies.

Although in this embodiment the number of filters for separatingcomponents having different frequencies from each other is two or four,the invention is not limited to such a case.

Embodiment 3

A third embodiment will be hereinafter described with reference to FIGS.12 and 13.

Referring to FIG. 12, a signal that is input to an input terminal 1201is input to a power amplifier 1204 via a combiner 1203. An output of thepower amplifier 1204 is branched into two signals by a divider 1205. Adistortion component is extracted from one branched signal by adistortion extraction circuit 1206. The amplitude frequencycharacteristic of the extracted distortion component is varied by anamplitude frequency characteristic adjustment circuit 1207 and theamplitude and the phase of a resulting distortion signal are varied by avector adjustment circuit l208. A resulting signal is input to thecombiner 1203 and thereby fed back to the input of the power amplifier1204. The other output of the divider 1205 is connected to an outputterminal 1202. A signal is taken out from the output terminal 1202. Alow distortion signal can be obtained by properly controlling theamplitude frequency characteristic adjustment circuit 1207 and thevector adjustment circuit 1208.

Highly accurate linearization can be attained in a stable manner bycontrolling the amplitude frequency characteristic adjustment circuit1207 and the vector adjustment circuit 1208 so as to minimize themagnitude of a distortion component extracted by the distortionextraction circuit 1206.

Having the amplitude frequency characteristic adjustment circuit 1207,the circuit of FIG. 12 is particularly effective in a case where theamplitude of a distortion signal generated actually by the poweramplifier 1204 is different from the amplitude of a distortion signalextracted by the distortion extraction circuit 1206.

A configuration of FIG. 13 is possible that is a circuit offrequency-separating a distortion signal included in a feedback signaland controlling the amplitude and the phase independently.

Referring to FIG. 13, a filter circuit 1307 selectively passes athird-order intermodulation distortion voltage on the low-frequency sideof a third-order intermodulation distortion generated by a poweramplifier 1304 and attenuates the other frequency components. A filtercircuit 1308 selectively passes a third-order intermodulation distortionvoltage on the high-frequency side by a power amplifier 1304 andattenuates the other frequency components.

Therefore, a distortion signal generated by the power amplifier 1304goes through a divider 1305 and is subjected to distortion extraction inthe distortion extraction circuit 1306. A low-frequency-side third-orderintermodulation distortion voltage passes through the filter circuit1307 and its amplitude and phase are varied by a vector adjustmentcircuit 1309.

A high-frequency-side third-order intermodulation distortion voltagepasses through the filter circuit 1308 and its amplitude and phase arevaried by a vector adjustment circuit 1310.

The two distortion signals are combined and then input to a combiner1303.

The low-frequency-side third-order intermodulation distortion voltage issuppressed by properly setting the amplitude variation amount and thephase variation amount of the vector adjustment circuit 1309, and thehigh-frequency-side third-order intermodulation distortion voltage issuppressed by properly setting the amplitude variation amount and thephase variation amount of the vector adjustment circuit 1310.

This configuration also provides an effect of suppressing a distortionthat occurs when a modulated wave is input.

Highly accurate linearization can be attained in a stable manner bycontrolling the vector adjustment circuits 1309 and 1310 so as tominimize the magnitude of a distortion signal extracted by thedistortion extraction circuit 1306.

A configuration shown in FIG. 14 is also possible.

Referring to FIG. 14, a distortion signal extracted by a distortionextraction circuit 1406 is frequency-separated by filter circuits 1407and 1408. The amplitude frequency characteristics of resultingdistortion signals are adjusted by respective amplitude frequencycharacteristic adjustment circuits 1409 and 1410, and then theiramplitudes and phases are adjusted by respective vector adjustmentcircuits 1411 and 1412. Resulting signals are input to a combiner 1403.

This configuration provides a distortion suppressing effect fordistortion signals having a plurality of frequencies.

Although in the above embodiment the number of filters for separatingcomponents having different frequencies from each other is two, theinvention is not limited to such a case.

Having the amplitude frequency characteristic adjustment circuits 1409and 1410, the circuit of FIG. 14 is particularly effective in a casewhere the amplitude of a distortion signal generated actually by thepower amplifier 1404 is different from the amplitude of a distortionsignal extracted by the distortion extraction circuit 1406.

Embodiment 4

A fourth embodiment of the invention will be hereinafter described withreference to FIG. 15. In FIG. 15, a predistortion circuit 1503 is one ofthe predistortion circuits of FIGS. 1, 7, 9, 10, and 11 according tofirst and second embodiments.

A signal that is input to an input terminal 1501 is given a distortionby the predistortion circuit 1503 and a resulting signal is input to apower amplifier 1504.

An output signal of the power amplifier 1504 is branched by a divider1505. One output signal of the divider 1505 is output from the circuitof FIG. 15 as a power-amplified signal. The other output signal of thedivider 1505 is input to a distortion amplitude/phase detecting means1506. A control means 1507 calculates optimum values of adjustmentpoints of the amplitude frequency characteristic adjustment circuit, thevector adjustment circuit, and the variable delay circuit (delay time)so as to minimize the magnitude of the distortion signal, and inputscalculation results to a control terminal 1508 of the predistortioncircuit 1503.

As a result, the adjustment points of the predistortion circuit 150.3such as those of the amplitude frequency characteristic adjustmentcircuit, the vector adjustment circuit, and the variable delay circuit(delay time) can reach, at high speed, optimum values that provides amaximum distortion suppressing effect.

For example, the amplitude and phase of a distortion can be detected byFourier-transforming a time-domain signal that is captured by a digitaloscilloscope into a frequency-domain signal and then calculating anamplitude and a phase of a distortion.

Embodiment 5

A fifth embodiment of the invention will be hereinafter described withreference to FIG. 16.

A signal that is input to an input terminal 1601 is branched into twosignals by a divider 1603. One branched signal is input to a poweramplifier 1606 via a vector adjustment circuit 1604 and a predistortioncircuit 1605 and amplified by the power amplifier 1606. The otherbranched signal is input to a combiner 1611 via a delay circuit 1610.

The signal that is input to the power amplifier 1606 is given adistortion component because of its nonlinearity. A resulting signal isbranched into two signals by a divider 1609. One branched signal isinput to a combiner 1617 via a delay circuit 1612.

The other branched signal produced by the divider 1609 is input to thecombiner 1611. The two signals that are input to the combiner 1611 arecombined together by the combiner 1611, whereby the carrier componentsare suppressed and only the distortion component is output.

The distortion component that is output from the combiner 1611 is inputto the combiner 1617 via a divider 1613, a vector adjustment circuit1615, and a power amplifier 1616.

The two signals that are input to the combiner 1617 are combinedtogether, whereby the distribution components are suppressed and asignal having no distortion components is output.

To obtain a sufficient carrier suppressing effect in the combiner 1611,it is necessary that the two routes from the divider 1603 to thecombiner 1611 have the same delay time and that the carrier componentshave the same amplitude and phases that are different from each other by180°.

To this end, the one branched signal produced by the divider 1613 isdetected by a signal level detecting means 1614. Resulting magnitudeinformation of the branched signal is sent to a control means 1620,which controls the pass amplitude and the pass phase of the vectoradjustment circuit 1604 so as to minimize the magnitude of the branchedsignal.

For the output of the combiner 1617 to be such that the distortion issuppressed sufficiently, it is necessary that that the two routes fromthe divider 1609 to the combiner 1617 have the same delay time and thatthe distortion components that are input to the combiner 1617 have thesame amplitude and phases that are different from each other by 180°.

To this end, the magnitude of the distortion level of the branchedsignal produced by a divider 1618 that is connected to the output of thecombiner 1617 is detected by a distortion level detecting means 1619.Resulting magnitude information is sent to the control means 1620, whichcontrols the vector adjustment circuit 1615 so as to minimize themagnitude.

The distortion generated by the power amplifier 1606 is branched by thedivider 1607. The magnitude of the distortion level is detected by adistortion level detecting means 1608 and resulting magnitudeinformation is sent to the control means 1620, which controls thepredistortion circuit 1605 so as to minimize the magnitude.

The pass amplitude and phase are varied when the predistortion circuit1605 is controlled, and hence the relationships between the amplitudesand the phases of the routes upstream of the combiner 1611 deviate fromoptimum ones, as a result of which a carrier component occurs in theoutput of the combiner 1611.

Therefore, if the control means 1620 controls the predistortion circuit1605 and the vector adjustment circuits 1604 and 1615 simultaneously, itmay not be able to perform controls so as to minimize the distortion.

In view of the above, first, the distortion level detecting means 1608detects a distortion signal generated by the power amplifier 1606 andthe control means 1620 controls only the predistortion circuit 1605 soas to minimize the level of the distortion signal. At this time, thecontrol means 1620 does not control the vector adjustment circuits 1604and 1615.

Then, the control means 1620 controls the vector adjustment circuit 1604so as to minimize the level detected by the signal level detecting means1614. At this time, the control means 1620 does not control thepredistortion circuit 1605 and the vector adjustment circuit 1615.

Then, the control means 1620 controls the vector adjustment circuit 1615so as to minimize the level detected by the distortion level detectingmeans 1619. At this time, the control means 1620 does not control thepredistortion circuit 1605 and the vector adjustment circuit 1604.

Performing the above three controls in arbitrary order makes it possibleto obtain maximum suppressing distortion.

The delay time of the predistortion circuit 1605 or the vectoradjustment circuit 1604 may vary when it is controlled. One method forcompensating for this variation in delay time is to employ, as the delaycircuit 1610, a variable delay circuit whose delay time is variable andcontrol it properly.

This is done in the following manner. A variation in delay time thatoccurs when the predistortion circuit 1605 or the vector adjustmentcircuit 1604 is controlled is measured in advance and stored in thecontrol means 1620. When the control means 1620 later controls thepredistortion circuit 1605 or the vector adjustment circuit 1604, itcontrols the variable delay circuit so as to compensate for the delaytime variation. The addition of this control enables suppressingdistortion of higher accuracy.

The distortion level detecting means may be a circuit that removes acarrier component with a notch filter and detects the magnitude of onlya distortion component.

Although in the first embodiment the output terminal 102 of thepredistortion circuit (see FIG. 1) is directly connected to the poweramplifier 204 (see FIG. 2) as a subject of linearization, the inventionis not limited to such a case; the former may be connected to the latterindirectly.

The above embodiments were mainly described for the case that theamplitude frequency characteristic adjustment circuit is providedupstream of the vector adjustment circuit (for examples, FIGS. 1, 7 and12). However, the invention is not limited to such case; the inventionis applicable to case where the amplitude frequency characteristicadjustment circuit is provided downstream of the vector adjustmentcircuit.

In the latter case, the predistortion circuit may be composed ofcomprising:

-   -   a divider for branching an input signal into plural branched        signals;    -   a delay circuit for delaying one of the branched signals by a        predetermined delay time;    -   a distortion generating circuit for receiving the other branched        signal and for generating a distortion signal;    -   a vector adjustment circuit for varying an amplitude and a phase        of the distortion signal; and    -   an amplitude frequency characteristic adjustment circuit for        varying an amplitude frequency characteristic of the distortion        signal that is output from the vector adjustment circuit;    -   a combining circuit for combining an output signal of the delay        circuit and an output signal of the vector adjustment circuit.

The above embodiments were mainly described for the case that both ofthe amplitude characteristics and the phase characteristics of thedistortion characteristics of the power amplifier as a subject oflinearization and the linearizer (for example, predistortion circuit)are different from each other. However, the invention is not limited tosuch a case; the invention is applicable to a case where only the phasecharacteristics are different from each other.

In the latter case, the predistortion circuit may be composed of adivider for distributing an input signal; a delay circuit connected toone output of the divider, for delaying a distributed input signal by aprescribed delay time; a distortion generation circuit connected to theother output of the divider; a vector adjustment circuit connected tothe output of the distortion generation circuit, for varying theamplitude and the phase of a distortion signal; and a combiner forcombining an output of the delay circuit with an output of the vectoradjustment circuit and for outputting a combined signal to a downstreamcircuit means as a subject of linearlization to be connected to thecombiner directly or indirectly. The delay time of the delay circuit isset based on a phase difference of a distortion that would be generatedby the circuit means if the vector adjustment circuit did not producethe above output.

A specific configuration is one obtained by removing the amplitudefrequency characteristic adjustment circuit 106 or 706 from thepredistortion circuit of FIG. 1 or 7. Also in this case, a control inaccordance with the degree of phase imbalance is enabled by adjustingthe delay time of the delay circuit, which is similar to the effectattained by each of the above embodiments.

The above embodiments were mainly described for the case that both ofthe amplitude characteristics and the phase characteristics of thedistortion characteristics of the power amplifier as a subject oflinearization and the linearizer (for example predistortion circuit) aredifferent from each other. However, the invention is not limited to sucha case; the invention is applicable to a case where only the amplitudecharacteristics are different from each other.

In the latter case, the predistortion circuit may be composed of adivider for distributing an input signal; a delay circuit connected toone output of the divider for delaying a distributed input signal by aprescribed delay time; a distortion generation circuit connected to theother output of the divider; an amplitude frequency characteristicadjustment circuit connected to the output of the distortion generationcircuit, for varying an amplitude frequency characteristic; a vectoradjustment circuit connected to the output of the amplitude frequencycharacteristic adjustment circuit, for varying the amplitude and phaseof a distortion signal; and a combiner for combining an output of thedelay circuit with an output of the vector adjustment circuit.

A specific configuration is similar to the predistortion circuit of FIG.1 and different from the latter in that the delay circuit 104 may bereplaced by a conventional delay circuit. Also in this case, a controlin accordance with the degree of amplitude imbalance is enabled byadjusting the amplitude frequency characteristic adjustment circuit inthe above-described manner, which is similar to the effect attained byeach of the above embodiments.

Although the above embodiments are directed to the case where thecircuit means as a subject of linearization is a power amplifier, theinvention is not limited to such a case. For example, the circuit meansas a subject of linearization may be a linear amplifier or other circuitdevice or circuit component.

As is apparent from the above description, the invention providesadvantages that a distortion of a power amplifier or the like that isunbalanced in amplitude or phase can be linearized for with a stablecontrol.

1. A low-distortion power amplifier comprising: a combining circuit forcombining an input signal with another signal; a power amplifier forreceiving an output signal of the combining circuit; a divider forbranching an output signal of the power amplifier into plural branchedsignals; a distortion extraction circuit for extracting a distortionsignal from one of the branched signals; at least two filter circuitsfor separating the distortion signal into distortion signals havingdifferent frequencies; at least two amplitude frequency characteristicadjustment circuits for adjusting amplitude frequency characteristics ofthe distortion signals that are output from the respective filtercircuits; and at least two vector adjustment circuits for varyingamplitudes and phases of distortion signals that are output from therespective filter circuits, wherein a signal obtained by combiningtogether output signals of the vector adjustment circuits is input tothe combining circuit as said another signal and the other branchedsignal is output from the low-distortion power amplifier.
 2. The controlmethod for the low-distortion power amplifier according to claim 1,comprising the steps of: detecting a magnitude of the distortion signalthat is output from the distortion extraction circuit; and controllingat least one of the amplitude frequency characteristic adjustmentcircuit or circuits and the vector adjustment circuit or circuits so asto minimize the detected magnitude of the distortion signal.